Lithium-ion secondary batteries are commonly used in small information devices such as mobile phones and notebook computers because they have a higher energy density than other types of secondary batteries such as nickel cadmium batteries and nickel hydride batteries, and can operate at high potential. Because it is easy to reduce their size and weight, there is growing demand for lithium-ion secondary batteries as secondary batteries in hybrid vehicles and electric vehicles. Because of high safety requirements in automotive applications, all-solid-state lithium-ion secondary batteries that do not use flammable electrolytes have been researched and developed. The solid electrolytes used in all-solid-state lithium-ion secondary batteries require high ion conductivity.
Materials having cubic garnet-related structures have been reported to have high ion conductivity (see, for example, Patent Document 1), and materials having this structure are currently being researched and developed. Materials with the chemical structure Li7−xLa3Zr2−xNbxO12 have been reported to have high ion conductivity near x=0.25. Because grain boundary resistance and interface resistance have to be reduced as much as possible to realize high ion conductivity, solid materials in the form of high-density compacts are desired. Solid materials in the form of high-density compacts also hold out the possibility of reducing the size of all-solid-state lithium-ion secondary batteries because they can prevent short circuits between the positive and negative electrodes during the charge and discharge cycle and can be produced in the form of slices. However, these materials with a garnet-related structure are difficult to sinter, so creation of high-density compacts is known to be difficult.
Garnet-related structures are commonly known to include tetragonal compounds in addition to cubic compounds (see, for example, Patent Document 1 and Non-Patent Document 1). However, solid lithium electrolytes having garnet-related structures with crystal systems that are not cubic or tetragonal have not been reported. Those with garnet-related structures are expected to have high lithium ion conductivity even when symmetry is low.